Dehydration and Dehydrated Vegetable Products

Dehydration, in food processing, means by which many types of food can be preserved for indefinite periods by extracting the moisture, thereby inhibiting the growth of microorganisms. Dehydration is one of the oldest methods of food preservation and was used by prehistoric peoples in sun-drying seeds. The North American Indians preserved meat by sun-drying slices, the Chinese dried eggs, and the Japanese dried fish and rice.

Many vegetables are available in dehydrated form. A typical process involves:

wet and dry cleaning

peeling, if necessary, by mechanical, steam or chemical methods

slicing, dicing or shredding

blanching

sulphiting , if necessary

dewatering

drying

conditioning, if used

milling or kibbling, if used

screening

packaging

Most vegetables are blanched prior to drying. Many vegetables are sulphured or sulphited prior to drying. Many different types of driers are used for drying vegetables including: cabinets, single or two stage tunnels, conveyor and fluidised bed driers .Air inlet temperatures vary from product to product but are usually in the range 50–110 _C. Tunnel, conveyor and fluidised bed driers may be divided into a number of drying zones, each zone being controlled at a different temperature, to optimize the process. If the drying is not completed in the main drier, the product may be finish dried or conditioned in a bin drier, supplied with dry air at 40–60 _C. Among the vegetables dried as outlined above are green beans, bell peppers, cabbage, carrot, celery, leeks, spinach and swedes. Some vegetables such as garlic, mushrooms, green peas and onions are not sulphited. Herbs such as parsley, sage and thyme may be dried without blanching or sulphiting. Vegetables may be dried in vacuum cabinet driers and in freeze driers, to yield products of superior quality to those produced by air drying. However, such products will be more expensive. Vegetable purees may be dried. Cooked and pureed carrot and green pea’s maybe drum dried to produce a flaked product. Very finely divided, cooked carrot or green peas may be spray dried to a fine powder.

A number of dried potato products are available. Dehydrated, diced potato is produced by a process similar to that outlined above. After blanching the potato pieces should be washed with a water spray to remove gelatinised starch from their surfaces. In addition to sulphite, calcium salts may be added to increase the firmness of the rehydrated dice. Cabinet, tunnel or conveyor driers may be used to dry the potato. Conveyor driers are most widely used. In recent years, fluidised bed driers have been applied to this duty. Finish drying in bins is often practiced. Potato flakes are produced by drum drying cooked, mashed potatoes. Two-stage cooking is followed by mashing or ricing. Sufficient sulphite is mixed with the mash to give 150–200 ppm in the dried product. An emulsion is made up containing, typically, monoglyceride emulsifier, sodium acid pyrophosphate, citric acid and an antioxidant and mixed into the mash. In some cases milk powder may also be added. The mash is then dried on single drum driers equipped with a feed roll and up to four applicator rolls. Steam at 520–560 kPa, absolute, is used to heat the drums. After drying, the dried sheet is broken up into flakes. Potato flour is made from poor quality raw potatoes which are cooked, mashed, drum dried and milled. Potato granules may also be produced from cooked, mashed potatoes. After cooking, the potato slices are carefully mashed. Some dry granules may be ‘added back’ to the mash, which then has sulphite added to give 300–600 ppm in the dried product. A second granulation stage then follows. It is important that as little as possible rupture of cells occurs at this stage. The granules are cooled to 15.5–26.5 _C and held at that temperature for about 1 h. During this ‘conditioning’ some retrogradaton of the starch occurs. A further gentle granulation then takes place and the granules are dried in pneumatic and/or fluidised bed driers to a moisture content of 6–7%.

Fig (1), Sun Dry Potatoes

Sliced tomatoes may be sun dried. The slices are exposed to the fumes of burning sulphur in a chamber or dipped in, or sprayed with sulphite solution before sun drying. In recent years, there has been an increase in demand for sun dried tomatoes which are regarded as being of superior quality to those dried by other means. Tomato slices are also air dried in cabinet or tunnel driers to a moisture content of 4% (wwb). The dried slices tend to be hygroscopic andsticky. They are usually kibbled or milled into flakes for inclusion in dried soup mixes or dried meals.

Fig (2), Sun Dry Sliced Tomatoes

Tomato juice may be spray dried. The juice is prepared by the ‘hot or cold break process and concentrated by vacuum evaporation up to 26–48% total solids content, depending on the preparation procedure, before it is spray dried. The powder is hygroscopic and sticky when hot and tends to adhere to the wall of the drying chamber. A tall drying chamber downward, concurrent flow may be used. Alternatively, a chamber with a shorter body featuring a downward, concurrent, spiral flow path may be used. The wall of the chamber is fitted with a jacket through which cool air is circulated, to reduce wall deposition. An air inlet temperature in the range 140–150_C is used and the product has a moisture content of 3.5%.